1. Field of the Invention
The invention relates generally to the field of photography, and particularly to a film cassette containing roll film. More specifically, the invention relates to a film cassette that is capable of automatically advancing a non-protruding film leader to the exterior of the cassette shell responsive to unwinding rotation of a film spool within the shell.
2. Description of the Prior Art
In conventional 35 mm film manufacturers' cassettes, such as manufactured by Eastman Kodak Co. and Fuji Photo Film Co. Ltd., the filmstrip is wound on a flanged spool which is rotatably supported within a cylindrical shell. A leading or forward-most section of the filmstrip approximately 2 1/3 inches long, commonly referred to as a "film leader", normally protrudes from a light-trapped slit or mouth of the cassette shell. One end of the spool has a short axial extension which projects from the shell, enabling the spool to be turned by hand. If the spool is initially rotated in an unwinding direction, the film roll inside the shell will tend to expand radially since the inner end of the filmstrip is attached to the spool, and the fogged leader section protruding from the slit will remain stationary. The film roll can expand radially until a firm non- slipping relation is established between its outermost convolution and the inner curved wall of the shell. Once this non-slipping relation exists, there is a binding effect between the film roll and the shell which prevents further rotation of the spool in the unwinding direction. Thus, rotation of the spool in the unwinding direction cannot serve to advance the filmstrip out of the shell, and it is necessary in the typical 35 mm camera to engage the protruding leader section to draw the filmstrip out of the shell.
A 35 mm film cassette has been proposed which, unlike conventional film cassettes, can be operated to automatically advance a film leader out of the cassette shell by rotating the film spool in the unwinding direction. The film leader originally is located entirely inside the cassette shell. Specifically, in U.S. Pat. No. 4,423,943, granted Jan. 3, 1984, there is disclosed a film cassette wherein the outermost convolution of the film roll wound on the film spool is radially constrained by respective circumferential lips of two axially spaced flanges of the spool to prevent the outermost convolution from contacting an inner curved wall of the cassette shell. The trailing end of the filmstrip is secured to the film spool, and the forward or leading end of the filmstrip is slightly tapered purportedly to allow it to freely extend from between the circumferential lips and rest against the shell wall at a location inwardly of a non-light-tight film passageway to the outside of the cassette shell. During initial unwinding rotation of the film spool, the leading end of the filmstrip is advanced along the shell wall until it reaches an inner entrance to the film passageway. Then, it is advanced into and through the film passageway to the outside of the cassette shell. The passageway has a width that is less than the width of the filmstrip, thus resulting in the filmstrip being transversely bowed as it is uncoiled from the film spool, and thereby facilitating movement of the film edges under the circumferential lips of the respective flanges. However, severe transverse bowing of the filmstrip in order to move its longitudinal edges under the circumferential lips may damage the filmstrip.
Like the type of film cassette disclosed in U.S. Pat. No. 4,423,923, commonly assigned U.S. Pats. No. 4,834,306, granted May 30, 1989, and U.S. Pat. No. 4,848,693, granted Jul. 18, 1989, each disclose a film cassette that is capable of automatically advancing a non-protruding film leader to the outside of the cassette shell in response to rotation of the film spool in the unwinding direction. Specifically, there is disclosed a film cassette wherein a film roll is wound on a spool between a pair of coaxially spaced, independently rotatable flanges. The two flanges have respective circumferential annular lips which prevent the outermost convolution of the film roll, including its leading end, from clock-springing into contact with the interior wall of the cassette shell. When the spool is initially rotated in the unwinding direction, the flanges may momentarily remain stationary and the film roll, since its inner end is secured to the spool, tends to expand radially to ensure a firm non-slipping relation between the outermost convolution and the annular lips. Once the non-slipping relation exists, continued rotation of the spool will similarly rotate the flanges. This allows stationary internal spreaders to deflect successive portions of the annular lips to an axial dimension exceeding the film width, in turn allowing the leading end and successive sections of the film roll to be freed from the radial confinement of the annular lips and to be advanced into and through a light-tight film passageway to the outside of the cassette shell. A stripper-guide is located adjacent an inner entrance to the film passageway to divert the leading end of the film roll into the passageway. To accomplish this, the stripper-guide must be received between the leading end and the next-inward convolution of the film roll. If, however, the film cassette is used during extremely high temperature and/or humidity conditions, it might be possible that the leading end after being freed from the radial confinement of the annular lips will retain a great deal of inward curl. Consequently, the leading end will not have sufficient clock-spring to separate sufficiently from the next-inward convolution to move over the stripper-guide. Thus, when the spool is rotated in the unwinding direction, the leading end may not be picked up by the stripper-guide and guided into the film passageway. Another problem is that the leading end may be slightly transversely bowed after being freed from the annular lips. Thus, when the leading end is moved over the stripper-guide, the leading end may stub at the inner entrance to the film passageway.